In recent years, distributed power sources utilizing natural energies and power storage systems including power storage means represented by a storage battery have been widely introduced. These distributed power sources and power storage systems internally include power storage devices and store electric power in the form of DC power. Electric power stored in a power storage device is converted into AC electric power having an interconnected power system frequency by a self commutated power converter, and sent out to the electric power system. In addition, power supplied by the electric power system is converted into DC electric power by the self commutated power converter, and stored in the power storage device.
A self commutated power converter includes an inverter that performs AC/DC conversion and a DC circuit, and the DC circuit includes a capacitor. Furthermore, a DC breaker is disposed between a distributed power source or a power storage system and the self commutated power converter, and while the self commutated power converter is in a halt state, the DC breaker is opened.
When the DC breaker is switched on and the distributed power source and the power storage system are connected to the self commutated power converter at the time when the self commutated power converter starts running, an excessive rush current is induced in the capacitor of the DC circuit, which could lead to the damages to these devices. This rush current arises owing to the potential difference between the voltage of the inverter side of the capacitor and the voltage of the distributed power source or the voltage of the power storage device of the power storage system, and the larger the potential difference between both terminals is, the larger the rush current is.
As one of measures against the above problem owing to the initial charging of the capacitor, Japanese Unexamined Patent Application Publication No. Hei8(1996)-126339 discloses a technique in which a switch and an electromagnetic switch having a resistor, both of which are connected in series, are disposed between a distributed power source or a power storage system and a self commutated power converter. The switch is switched on at the time when the self commutated power converter starts running, and the distributed power source or the power storage system is connected to the self commutated power converter via the resistor, and subsequently the electromagnetic switch having the resistor are switched on. Therefore, when the switch is switched on, a current limited by the resistor flows through the capacitor, which decreases the potential difference between both terminals, with the result that an excessive rush current can be prevented from occurring.
According to Japanese Unexamined Patent Application Publication No. Hei8 (1996)-126339, although it is possible to decrease the potential difference between both terminals using a limiting resistor, it is necessary to install a switch and an electromagnetic switch having a resistor, therefore there arises a problem in that the number of necessary parts are increased, and the power converter grows larger and more complex.